rhg: add rhg crate The goal of rhg is to speedup some of hg's commands when possible by bypassing python entirely for the time being. It is by no means a replacement for hg as it will not support extentions or configuration and implement only a subset of hg's commands and options. Only use rhg if you understand what the tradeoffs are. Differential Revision: https://phab.mercurial-scm.org/D8610